The osmosis indicates the phenomenon of diffusion of Molécule S of solvent (water in a general way) through a semipermeable membrane which separates two liquids from different concentrations in aqueous solution. The solvent passage from one compartment to another will create a hydrostatic difference of Pression which will compensate for exactly the difference in pressure osmotic (cf hereafter).

This concept made it possible to better include/understand the behavior of the aqueous solutions in Chimie, at the end of the 19th century; but it is also particularly useful in Physiologie and cellular Biologie to explain the chemical exchanges within the living organisms.

History

In 1748 or 1749, the abbot Nollet notices that when one separates from the Eau and the alcohol by a animal Vessie, water passes in alcohol but never the reverse. In its work on the aqueous solutions carried out between 1827 and 1832, Rene Dutrochet proposes the terms “of endosmose” and “exosmose” to indicate this phenomenon. K. Vicrordt also is interested in this phenomenon in 1848. In 1854, Thomas Graham works on the substances Colloïde S and discovers that they cannot pass an animal membrane.

It is Mr. Traube, in 1864, which designs the first copper artificial ferrocyanide membrane Cu₂Fe (CN) ₆. In 1877, Wilhelm Friedrich Philipp Pfeffer (1845-1920) made precipitate the Ferrocyanure of Cuivre in a porous material, which makes it possible to have a membrane with a good mechanical resistance.

In 1884, of Vries works on the Plasmolyze and the Turgescence of the vegetable cells.

In 1886, van 'T Hoff publishes an analogy between the aqueous solutions and the Perfect gas S and applies the Thermodynamique to osmosis. It establishes a law similar to the Loi of Gay-Lussac and proposes the “semipermeable” adjective to indicate the membranes. It receives the Nobel Prize of chemistry in 1901 for its work.

In 1899, A. Crum Brown uses three liquid phases (an aqueous solution of Nitrate of Calcium saturated with phenol in bottom, layer of pure phenol in the medium and a water solution saturated with phenol in top). He notices a phenomenon of osmosis (water passes from the phase top towards the phase of bottom), the liquid phase of the medium playing the part of semi-porous membrane. He thus establishes the importance of the solubility of the species diffusing in the membrane.

Between 1901 and 1923, H. NR. Morse and J.C.W. Frazer undertake a systematic work of measurement of the permeability for various gelatinous precipitates: ferrocyanides and Phosphate S of Uranyl, Iron, Zinc, Cadmium and Manganese.

; Bibliography

S. Glasstone, Textbook off physical chemistry 2nd ED. (1948), Macmillan Student Edition

Phenomenon

One highlights osmosis by the passage of Molécule S or Ion S through a membrane, when there are two solution S of composition different on each side.

The phenomenon of osmosis requires the presence of two compartments separated by a semipermeable membrane, i.e. permeable only with water (or with solvent in a more general way) and impermeable with the aqueous solutions. When the two solutions do not contain the same number of dissolved particles per unit of volume, one observes a water movement which will try to compensate for this difference in concentration by diluting the compartment more concentrated.

Osmosis is at the origin of the Turgescence and the Plasmolyze of the vegetable cell.

Chemical potential

Ideal solution the reduction in chemical potential correspond to:

$RT\; \backslash \; ln\; (1\; -\; x\_2)\; \backslash \; qquad\; (1)$

where $R$ is the constant Gas, $T$ is the temperature and $x\_2$ is the aqueous solution concentration in terms off Mole fraction. Ideal Most real solutions approximate behavior for low aqueous solution concentrations (At higher concentrations interactions between aqueous solution and solvent cause deviations from Equation 1). This reduced potential creates has off driving force and it is this force which drives diffusion toilets through the semipermeable membrane. -->

Osmotic pressure

The osmotic pressure , is defined as the Pression minimum which should be exerted to prevent the passage of a solvent of a solution less concentrated with a solution more concentrated through a semi-porous membrane (Membrane hémiperméable). In biophysics, one distinguishes the oncotic pressure which is the share of the osmotic pressure due to proteins.

The osmotic pressure is proportional to the concentrations of aqueous solution on both sides of the membrane and the temperature; when one is in the presence of several aqueous solutions, it is necessary to take into account the totality of the aqueous solutions (with the manner of a composed gas, nap of the partial pressures).

The osmotic pressure of a Ideal solution is calculated by a formula developed by van' T Hoff in 1886 and applying the Second principle of thermodynamics.

$\backslash \; pi\; \backslash \; cdot\; V\; =\; -\; R\; \backslash \; cdot\; T\; \backslash \; cdot\; ln\; (1\; -\; f\_s)$

where

• $\backslash \; Pi$ is the osmotic pressure, in Pa;
• $V$ is the molar volume occupied by solvent;
• $R$ is the Constante perfect gases;
• $T$ is the absolute Température, in K;
• $f\_s$ is the molar Fraction aqueous solution.

The equation applied to the real solutions is, as for it,

$\backslash \; pi\; \backslash \; cdot\; V\; =\; -\; R\; \backslash \; cdot\; T\; \backslash \; cdot\; ln\; (1\; -\; \backslash \; gamma\; f\_s)$

where $\backslash \; gamma$ is the coefficient of activity aqueous solution.

For a very diluted solution, $f\_s$ is close to 0, and $-\; ln\; (1\; -\; f\_s)$   ≈  $f\_s$. One can thus simplify the equation in

$\backslash \; pi\; =\; \backslash \; frac\; \{f\_s\; \backslash \; cdot\; R\; \backslash \; cdot\; T\}\; \{V\}\; =\; C\; \backslash \; cdot\; R\; \backslash \; cdot\; T$: it is the law of van' T Hoff

where

• $c$ is the concentration of the solution (by summoning all the species present).

One can also write it like this:

$\backslash \; pi\; =\; R\; \backslash \; cdot\; T\; \backslash \; cdot\; I\; \backslash \; cdot\; M$

where

• $i$ is the number of particles by entity form
• $M$ is the molar Concentration (moles per liter)

(One calls $i\; \backslash \; cdot\; M$ the molar concentration colligative)

One sees the analogy with the law of the Perfect gas S

$p\; \backslash \; cdot\; V\; =\; N\; \backslash \; cdot\; R\; \backslash \; cdot\; T$

where

• V is the volume of gas;
• N is the number of moles of gas;

Let us consider two aqueous solutions 1 and 2 of osmotic pressures $\backslash \; Pi\_1$ and $\backslash \; Pi\_2$, then if $\backslash \; Pi\_1$ > $\backslash \; Pi\_2$, water passes from 2 towards 1; 2 concentrates ($\backslash \; Pi\_2$ increases) and 1 is diluted ($\backslash \; Pi\_1$ decreases), until equality between the osmotic pressures.

Osmotic pressure and hydrostatic pressure

The osmotic pressure is also a mechanical pressure, exerting a force on the membrane. If the difference in pressure osmotic is very large, that can involve the rupture of the membrane (case of the Hémolyse).

Contrary, if one exerts a mechanical pressure (Hydrostatique), one can force the passage of species through the membrane. It is what arrives at the time of a acute edema of the lung, and it is what one uses in the Osmose reverses. This phenomenon is also observable on eggs of the fish of aquarium which can burst or be crushed according to the difference in pressure osmotic on each side of the membrane, the shell.

The Osmose reverses is a technique of purification of water; it is also a technique of Dessalement of sea water allowing the production of fresh water.

Notes and references of the article

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